This disclosure pertains to reticles and masks as used in microlithography, especially reticles and masks as used in charged-particle-beam microlithography. More specifically, the disclosure pertains to xe2x80x9cmask blanksxe2x80x9d from which actual pattern-defining reticles and masks are made, and to methods and apparatus for measuring internal stresses of membrane regions of segmented mask blanks and the like having multiple membrane regions.
In recent years, miniaturization of active circuit elements in various microelectronic devices has proceeded at a rapid pace. This development of progressively smaller circuit elements has required the parallel development of ever more sophisticated apparatus and methods for manufacturing microelectronic devices including such circuit elements.
Progress in microelectronic-device-fabrication technology is exemplified by progress in microlithographic exposure technology, by which circuit patterns are imprinted on any of various xe2x80x9csubstratesxe2x80x9d (typically a semiconductor wafer). As the limitations of optical microlithography have been increasingly apparent, considerable attention has been devoted to producing a practical xe2x80x9cnext generationxe2x80x9d microlithography technology. Most of this effort has focused on the use of a charged particle beam (e.g., electron beam or ion beam) or a xe2x80x9csoft X-rayxe2x80x9d beam. In either of these approaches, the microlithographic pattern to be transferred to the wafer or other substrate is defined on a xe2x80x9creticlexe2x80x9d or xe2x80x9cmaskxe2x80x9d that is usually segmented to include multiple membranous regions that define respective portions of the pattern. (In this disclosure, the terms xe2x80x9cmaskxe2x80x9d and xe2x80x9creticlexe2x80x9d are used interchangeably; usually the term xe2x80x9cmaskxe2x80x9d is used.)
In a conventional method for fabricating a mask, semiconductor-processing technology is used. The method begins with a silicon substrate as a starting material, from which a xe2x80x9cmask blankxe2x80x9d (mask that does not yet define a desired pattern) is produced. A representative segmented mask blank 30 is shown in FIGS. 3(a)-3(b), wherein FIG. 3(a) is a plan view and FIG. 3(b) is an enlarged elevational section. As shown in FIG. 3(b), the mask blank 30 comprises a base material 31 (typically silicon), a plurality of struts 32 (typically made of the same material as the base material 31), and at least one thin-film membrane 33 supported by the base material 31 and struts 32. Note that the struts 32 divide the mask blank 30 into multiple xe2x80x9csubfieldsxe2x80x9d characteristic of a segmented mask blank, wherein the membrane 33 extends over each subfield between the struts. When the mask blank 30 is made into an actual mask, the membrane region of each subfield is configured with a respective portion of a desired mask pattern. The mask pattern usually is of a circuit pattern to be imprinted as a respective layer on a lithographic substrate.
Referring further to the mask blank 30, whenever the residual stress of the membrane 33 is excessive, pattern distortion occurs after the mask blank is made into a mask. Hence, it usually is necessary to ascertain the magnitude of residual stress in the membrane 33 before or during manufacture of a mask from the mask blank.
Conventional methods for measuring stress and Young""s modulus in a membrane include so-called xe2x80x9cbulgexe2x80x9d techniques. Bulge techniques are advantageous because they can provide stress measurements without damaging the membrane. An overview of a bulge technique is shown in FIG. 4. The depicted bulge technique involves mechanically securing the peripheral portions of a membrane 42 using a membrane-holding plate 44. The membrane 42 is placed over an opening in a pressurization chamber 41. While varying the pressure applied via the chamber 41 to the membrane 42, the magnitude and direction of membrane bulge is measured using a bulge-measuring device 43. From the bulge measurements and from corresponding pressure values, the membrane stress and the Young""s modulus of the membrane 42 are determined.
The relationship between pressure applied to the membrane 42 and magnitude of bulging of the membrane 42 can be expressed as in Equation (1):
Pxc2x7[r2/(dxc2x7h)]=K1xc2x7"sgr"+K2xc2x7[E/(1xe2x88x92xcex3)]xc2x7(h/r)2xe2x80x83xe2x80x83(1) 
wherein P is the pressure applied to the membrane 42, "sgr" is the stress of the membrane 42, E is Young""s modulus, xcex3 is Poisson""s ratio, r is the membrane radius (if the membrane 42 is circular) or one-half the length of one side (if the membrane is square), d is the thickness of the membrane 42, h is the bulge of the membrane, and K1 and K2 are constants determined according to the shape of the membrane 42.
Thus, using conventional bulge techniques it is possible to measure the stress of the membrane of a mask blank so long as the peripheral portions of the membrane are adequately secured for making the measurements. Unfortunately, however, masks used in actual charged-particle-beam and soft X-ray microlithographic exposure apparatus are segmented and thus have multiple subfields. Each subfield has a respective membrane supported by flanking struts on a support substrate. With such a mask blank it is necessary to control and secure the peripheral portion of an individual subfield membrane in order to perform the measurements. Conventional methods and apparatus simply do not provide the requisite level of control and security for obtaining measurements at the required accuracy.
Furthermore, in an actual segmented mask blank, the center-to-center distance between adjacent subfield membranes is very small (e.g., about 1 mm), and the width of individual struts is even smaller, typically several hundred xcexcm. Such a mask blank is too weak to withstand mechanical clamping. I.e., it is extremely difficult to hold and secure the mask blank securely without causing damage. Another problem with conventional methods and apparatus (in which the periphery of the mask blank is clamped) is that application of pressure to the mask blank causes the entire unclamped region of the mask blank to bulge, which tends to fracture the membranes.
Therefore, obtaining accurate measurements of stress in the membranes of mask blanks (especially segmented mask blanks) currently is extremely difficult.
In view of the difficulties with conventional techniques for measuring residual stress in the membrane of a mask blank, the present invention provides, inter alia, methods and apparatus for more accurately measuring membrane stress, especially of a segmented mask blank having a plurality of membranous subfields such as would be used to fabricate a mask for use in charged-particle-beam or soft X-ray microlithography.
According to a first aspect of the invention, methods are provided for measuring stress in membrane regions of a segmented mask blank defining multiple subfields each having a respective membrane region flanked by struts that separate the subfields from one another. In an embodiment of such a method, the mask blank is mounted on a securing plate such that the struts contact the securing plate. The securing plate defines an array of through-holes, wherein the array has a pitch substantially equal to the pitch of subfields of the mask blank. The mask blank is situated on the securing plate such that the through-holes are aligned with individual respective subfields of the mask blank. A differential pressure is applied across the respective membrane regions of subfields aligned with respective through-holes. Respective displacements of the membrane regions to which the differential pressure is being applied are measured. From the respective displacements, respective values of membrane stress of the membrane regions are determined based on a relationship between the pressure and magnitude of membrane displacement.
The differential pressure can be applied in a manner that causes bulging of the membrane regions, or that causes indentation of the membrane regions.
The relationship between pressure and membrane displacement can be as expressed above in Equation (1).
Because the struts of the mask blank are held to the securing plate by electrostatic force rather than mechanical clamping, it now is possible to secure the respective periphery of each of multiple subfields firmly, regardless of the distance between adjacent struts or of the weakness of the mask blank. Also, the individual through-holes in the securing plate allow pressure to be applied selectively to respective membrane regions of individual subfields. As a result, it now is possible to measure stress in the membrane regions of individual subfields without such measurements being influenced by measurements obtained at other subfields.
According to another aspect of the invention, apparatus are provided for measuring stress in membrane regions of a segmented mask blank defining multiple subfields each having a respective membrane region flanked by struts that separate the subfields from one another. An embodiment of such an apparatus comprises a securing plate defining an array of through-holes, wherein the array has a pitch substantially equal to the pitch of subfields of the mask blank. As noted above, the securing plate is configured for placing the mask blank thereon such that the struts contact the securing plate and the through-holes are aligned with individual respective subfields of the mask blank. The apparatus also includes a means for holding the mask blank to the securing plate. (In embodiments in which the mask blank is held electrostatically to the securing plate, this means can include a power supply connected at least to the securing plate and configured to generate an electrostatic force sufficient to attract the mask blank to the securing plate.) The apparatus also includes a mechanism for applying a differential pressure across the respective membrane regions of subfields aligned with respective through-holes. The apparatus also includes a device for measuring respective displacements of the membrane regions to which the differential pressure is being applied.
The mechanism for applying a differential pressure desirably comprises a chamber and a device, connected to the pressurizing chamber, configured for creating a pressure inside the chamber relative to outside the chamber, the chamber defining an opening. In this configuration, the securing plate extends across the opening in the chamber.
The device for measuring respective displacements can be one or more of the following: (1) an instrument that employs a stylus-type displacement gauge, (2) an instrument that observes the membrane using a microscope for determining changes in focal position of various loci (e.g., center versus periphery) on the membrane, (3) an instrument that irradiates a light onto the membrane and measures membrane displacement by measuring changes in interference fringes of light reflected from the membrane versus from a reference surface, and (4) an instrument that irradiates a light onto the membrane and measures displacement of a position of light reflected from the membrane.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.